In many applications, nonvolatile, secondary memory is achieved by using magnetic disk recording. While areal density improvements are underway in magnetic recording, the fact that they are mechanical systems limits their reliability, volumetric data storage capacity, access time, data rate, and usefulness in harsh environments such as in spaceflight.
U.S. Pat. No. 4,660,173 and published reports describe a means for achieving three-dimensional storage using magnetic bubble technology for storage and magneto-optic sensing for readout. These published reports are the following:
1. F. Mehdipour, U.S. Pat. No. 4,660,173, "Three Dimensional Magnetic Bubble Data Storage and Optical Retrieval System.";
2. A. J. Mendez and F. Mehdipour, "Three Dimensional Mass Memory Compatible with Parallel Processing Architectures," Accepted for publication, IEEE Fourth Annual Symposium on Parallel Processing;
3. "Firm Developing optical Media Based on Magnetic Bubble Technology." Optical Memory News, October, 1989, Rothchild Consultants, San Francisco;
4. F. Mehdipour and H. Bagherzadeh, "New Memory Technology Supports Parallel Processing," Defense Electronics, Volume 22, Number 11, November 1990, pp. 55-58.
However problems exist in the two methods that comprise the current state of the art. First, in one method, the optical path is common along one axis to the storage layers. Such a system is simple, and uses the Faraday effect, as depicted in FIG. 1, in which the polarization of the incident, linearly polarized light is rotated in one sense or the opposite, depending upon the direction of magnetization. However, as shown in FIG. 2, since only the net Faraday rotation can be measured at an output, the data at each of the bit locations along the optical path cannot be uniquely recovered. Thus, proper readback cannot occur as described in the prior art.
A second method in the prior art places a single magnetic bubble layer between two layers which serve as optical waveguides. Such an arrangement allows data to be recovered uniquely since unique optical paths are provided, but this capability is achieved at the expense of considerable material and processing complexity. This additional complexity may degrade memory performance to an unacceptable level since the physical stresses and temperature dependencies encountered during device processing for such a module are significant. The relatively high temperatures needed to fabricate the optical waveguide layers are likely to stress and alter the magnetic bubble storage material, which often is an epitaxially grown garnet crystal containing a variety of rare-earth constituents and is fabricated using relatively low-temperature processes. These problems can change the anisotropy, mobility, magnetization, characteristic length, etc. of the bubble film, and affect the operational storage and propagation functions. (See for example P. H. L. Rasky, D. W. Greve, and M. H. Kryder, "The Feasibility of Silicon on Garnet Technology," Journal of Applied Physics, Volume 57, Number 1, Apr. 15, 1985, pp. 4077-4079) A general text in the relevant art is " Magnetic Bubble Technology" by A. H. Eschenfelder, 2nd Edition, 1981 published by Springer-Verlag, Berlin/New York.